Non air-polluting corona discharge devices

ABSTRACT

A corona generating device, adapted to operate in air, to apply an electrostatic charge to a surface, includes an elongated, knife-edged, corona-generating electrode enclosed within a housing. The housing includes a foraminous screen that has an optical transmission of less than 50 percent to prevent ultraviolet light produced by the corona from reaching the surface to be charged. The screen and the interior of the housing are formed, or coated, with a catalytic material, such as silver, to convert any ozone formed by the corona discharge, to oxygen, thereby preventing pollution of the air with toxic ozone.

United States Patent Kiess [451 July 4, 1972- [54] NON AIR-POLLUTINGCORONA DISCHARGE DEVICES [72] Inventor: IIelmut Gustav Kiess,Hightstown, NJ.

[73] Assignee: RCA Corporation [22] Filed: April 2, 1971 21 Appl. No.:130,644

[52] US. Cl. ..317/262 A, 250/495 ZC, 250/495, GC, 317/2 R, 317/2 F,317/262 AE [51] Int. Cl. ..H01t19/00 [58] Field of Search ..317/262 A,262 AB, 2 F, 2 R; 204/176, 313; 250/495 ZC, 49.5 GC

[56] References Cited UNITED STATES PATENTS 3,470,417 9/1969 Gibbons...250/49.5 ZC X 3,566,108 2/1971 Weigl et al "317/1262 A X 3,557,367l/l97l Roth ..3l7/262 A X OTHER PUBLICATIONS Guide to Corona FilmTreatment by von der Heide & Wilson, May 1961, issue of Modern Plastics,p. 200

Primary Examiner--.]. D. Miller Assistant Examiner-Harry E. Moose, Jr.AttorneyGlenn H. Burestle [5 7] ABSTRACT 7 Claims, 3 Drawing FiguresBACKGROUND OF THE INVENTION This invention relates generally toapparatus, adapted to operate in air, for applying an electrostaticcharge to a surface; and, more particularly, to non air-polluting coronadischarge devices. The improved corona discharge devices of the presentinvention are particularly useful for electrostatically chargingrecording elements employed in the electrostatic printing art.

I In electrophotographic printing processes in common use it isnecessary to apply a uniform electrostatic charge, in darkness, to thesurface of a photoconductive layer of a recording element. When therecording element is exposed to a light image, developed with apigmented toner, and fixed, it provides a copy of the image to which itwas exposed. It has been proposed to charge the surface of the recordingelement electrostatically by subjecting its photoconductive surface to acorona discharge, in air, from a corona discharge device disposed abovethe photoconductive surface. While such a corona discharge device issuitable for charging a photoconductive surface for many practicalpurposes, it produces an ultraviolet glow which adversely affectsuniform charging. Also, the production of a corona discharge in airusually generates ozone, which also adversely affects uniform chargingand which, even in very small quantities, is considered to be toxic tohumans. A concentration of 1 part ozone in 20,000 parts of air irritatesmucous membranes, when breathed, and is poisonous.

l have observed that ultraviolet light and ozone, both formed during acorona discharge, discharge an electrostatically chargedelectrophotographic recording element. In fact, I have observed thatozone alone, in the absence of light, discharges a chargedphotoconductive layer of zinc oxide in a resin binder very quickly,actually in a matter of about one second. It is believed that thespurious discharges of a charged photoconductive layer of a recordingelement, caused by both the ozone and the ultraviolet light during anelectrophotographic process, produce unwanted, spurious toning defectsin the finished electrophotographic print. This is especially observablein reversal prints wherein the spurious discharges cause unwanted tonedspots in background (white) areas of the prints. The spurious toningdefects degrade the quality of the finished electrophotographic print.

SUMMARY OF THE INVENTION The novel apparatus for applying anelectrostatic charge to a surface comprises a housing, having aforaminous screen as a portion thereof. The screen is adapted to bedisposed adjacent the surface to be charged. A corona generatingelectrode is enclosed within the housing and insulated therefrom. Theelectrode is disposed adjacent to the screen and is adapted to generate,when energized, a corona discharge in the general direction of thescreen. A catalyst is disposed within the housing to convert any ozonethat may be formed during the generation of the corona discharge tooxygen, thereby preventing toxic ozone from polluting the atmosphere.

In a preferred embodiment of the novel corona discharge device, thescreen has an optical transmission of less than 50 percent, and a baffleplate disposed between the electrode and the surface to be charged toshield the surface from both ultraviolet light and ozone.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view, inside elevation, of one embodiment of a non air-polluting coronadischarge device disposed to apply an electrostatic charge to anelectrophotographic recording element;

FIG. 2 is a cross-sectional view taken along the line 2-2 in FIG. 1; and

FIG. 3 is a cross-sectional view, in side elevation, of anotherembodiment of a non air-polluting corona discharge device disposed toapply an electrostatic charge to a recording element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIGS. 1 and 2,there is shown, in cross-section, a non air-polluting corona dischargedevice 10 for applying an electrostatic charge to a surface 12 of aphotoconductive layer 14 of a recording element 16. The recordingelement 16 is of the type commonly used in the electrostatic printingart. The recording element 16 comprises a relatively electricallyconductive substrate 18, such as paper, and the photoconductive layer14, such as of zinc oxide in a suitable resin binder thereon. Therecording element 16 is on an electrically conductive support plate 20,disposed beneath the corona device 10, and adapted to be moved, by anysuitable means, in the direction of the arrow 52.

The corona discharge device 10 comprises an enclosed, electricallyconductive housing 22, formed by a downwardly extending shield 24, ahorizontally extending foraminous screen 26, a horizontally extendingbaffle plate 28, and an upwardly extending foraminous screen 30. Thepositional orientation described herein is merely relative with respectto the horizontally disposed recording element 16. It is within thecontemplation of the present invention, however, to orient the recordingelement 16 and the corona discharge device 10 in positions other thanhorizontal. The screens 26 and 30 are fixed to the shield 22 and to thebafile plate 28 by any suitable means, as, for example, by welding orsoldering.

Means are provided to generate a corona discharge, that is, a source ofions, when energized by a suitable power supply. To this end, anelongated, corona generating electrode 32 is disposed within theinterior 34 of the housing 22. The electrode 32 is an elongated band, orstrip, formed with a knife edge 36 pointing in the general direction ofthe screen 26, for the purpose hereinafier appearing. The electrode 32is insulated from the conductive baffle plate 28 by insulating threadedstuds 38 that extend upwardly from the baffle plate 28. The studs 38 maybe adhered to the baffle plate 28 by any suitable means, such as byepoxy glue, and the electrode 32 may be adjustably mounted on thethreaded studs 38 with nuts.

The screens 26 and 30 should have an optical transmission of less than50 percent. The term optical transmission, as used herein, is defined asthe ratio of the area of the holes to the area of both the holes and thenon-holes of the screen, considering the screen as a two dimensionalplanar object. Thus, the optical transmission of a foraminous screen isessentially the percentage of light it will transmit when exposed tolight. I have found that the optical transmission of the screen 26should be less than 50 percent to prevent both ultraviolet light andozone that may be formed during the generation of a corona dischargefrom passing from the interior 34 of the corona discharge device 10 ontothe surface 12 of the recording element 16. Also, the electrode 32 andthe baffle plate 28 are disposed with respect to each other so that thebaffle plate 28 prevents ultraviolet light, emanating from the knifeedge 36 of the electrode 32, from reaching the surface 12 of therecording element 16. Thus, the bafile plate 28 is disposed between thesurface 12 of the recording element 16 and the electrode 32. Thejunction 40, between the baffle plate 28 and the screen 26, is disposedso that any ultraviolet light emanating from the knife edge 36 of theelectrode 32 does not reach the surface 12 to be charged.

Means are provided to energize the corona discharge device 10. To thisend, the negative terminal of a unidirectional power supply 42,represented symbolically herein as a battery, is connected to theelectrode 32 by a conductor 44 that passes through an insulator 46positioned in the screen 30. The positive terminal of the power supply42 is connected to a point of reference potential, such as ground. Theshield 24 of the housing 22 is connected to ground through a variableresistor 48 so that the potential of the shield 24 may be biased withrespect to the positive terminal of the power supply 42. The supportplate 20 is connected to ground through a variable voltage source 50,the positive terminal of the voltage source 50 being connected to theplate 20 and the negative tenninal being grounded. Thus, the substrate18 of the recording element 16 can be biased positively with respect tothe electrode 32 and the housing 22 to cause a negative corona dischargeto be attracted to the positively biased surface 12 of the recordingelement 16.

A negative electrostatic charge is applied to the surface 12 of therecording element 16 by the corona discharge device 10 as follows: Anegative voltage of at least 5,000 volts with respect to ground isapplied to the electrode 32. The variable resistor 48 is adjusted sothat the voltage of the housing 22 is between and 500 volts with respectto ground. The variable voltage source 50 is adjusted so as to provide avoltage to the support plate 20 of between about 0 and +400 volts withrespect to ground. The support plate 20 should be maintained at apositive potential with respect to the housing 22. The corona dischargedevice 10 is disposed so that the bafile plate 28 and screen 26 areparallel to, and about mm above, the

surface 12 of the recording element 16. Under these conditions, anegative corona discharge enanates from the knife edge 36 of theelectrode 32 in the direction of, and through, the screen 26. Theelectric wind produced by the corona discharge blows the negative ionson the corona through the screen 26 and onto the surface 12 of therecording element 16.

The recording element 16 is usually adapted to be moved in the directionof the arrow 52. It is also within the contemplation of the presentinvention to move the corona discharge device with respect to either afixed or a moving recording element 16. Any radiation in the form oflight that may emanate from the knife edge 36 of the electrode 32 duringthe generation of the corona discharge is prevented from reaching thesurface 12 by both the bafile plate 28 and by thescreen 26 (because ofits limited optical transmission of less than 50 percent).

I have observed that ozone is generated within the corona dischargedevice 10 during the generation of the corona discharge. I have alsoobserved that ozone, alone' and in darkness, discharges a chargedrecording element 16. It is also well known that ozone, even in therelatively small quantities of 50 parts of ozone to 1 million parts ofair, is toxic to humans. An important feature of the novel coronadischarge device 10 is its provision for preventing, or substantiallyreducing, the pollution of the atmosphere by the ozone formed during thegeneration of the corona discharge. To this end, the inner surface ofthe shield 24 and the inner surface of the baffle plate 28 are coatedwith a catalytic coating 54 of a catalyst for converting to oxygen anyozone that may be formed. I have found silver to be substantially thebest coating 54 for catalyzing the chemical reaction:

Ag Catalyst 30 oxygen ozone Other suitable catalysts for the catalyticcoating 54 are gold, nickel, platinum, iron, and their alloys. Certainoxides are also suitable catalysts. These are, for example, nickelsesquioxide (Ni O barium'monoxide (BaO), cupric oxide (CuO), and silverperoxide (Agg02)- Some of these metal oxides may be formed by firstplating the shield 22'and the bafile plate 28 with a layer of the metalof the oxide and then oxidizing the deposited metal layer to form therequired metal oxide. For example, a layer of nickel when heated at atemperature of between 400 and 480 C. is converted partially to thenickel sesquioxide (NigOa). Cupric oxide may be formed, for example, byheating a copper coating in air until the surface of the copper turnsblack.

The catalytic coating 54 of platinum can be formed by coating theinterior of the corona discharge device 10 with platinum, treating thecoating with aqua regia having an excess of nitric acid, evaporating theacid, and heating the treated surface until it turns black. The screens26 and 30 are also either coated with, or comprise, at least one of theaforementioned catalytic materials. Hence, ozone formed within theinterior 34 of the corona discharge device 10 is acted upon by thecatalytic coating 54 and the catalytic material on the screens 26 and 30to convert the ozone to oxygen.

Referring now to FIG. 3, there is shown an improved corona dischargedevice 60 which differs from the corona discharge device 10 in that thecorona discharge device 60 does not have a baflle plate to function as alight shield. The corona discharge device 60, however, employs a screen62, similar in structure and function to the screen 26 of the coronadischarge device 10 with the exception that the screen 62 may have anoptical transmission that is less than that of the screen 26. Thus, theoptical transmission of the screen 62 is between about 20 percent and 50percent. Parts of the corona discharge device 60 in FIG. 3 that aresimilar to those of the corona discharge device 10 in FIG. 1 in bothstructure and function are given the same reference numerals.

The corona discharge device 60 comprises an enclosed housing 64 of whichthe screen 62 is a part. The housing 64 comprises a rectangular box-likestructure having side walls 66 and 68 and end walls (not shown). A topwall 70 of the housing .64 comprises a screen 70 similar in structureand function to the screen 30. An elongated electrode 72 is disposedwithin the interior 74 of the housing 64. The electrode 72 is supportedwithin the interior 74 by electrically insulating rods 76 and isdisposed so that a knife edge 78 of the electrode 72 causes a coronadischarge therefrom to flow in the general direction of the screen 62and onto the surface 12 of the recording element 16.

The corona discharge device 60 is energized from a source 42 ofunidirectional voltage, the negative terminal of the voltage source 42being connected to the electrode 72 through an insulator 80 in the wall66, the positive terminal being connected to a point of referencepotential, ground. The housing 64 is connected to ground through avariable resistor 48 to bias it negatively with respect to ground. Thesupport plate 20 is connected to the positive terminal of a voltagesource 50, the negative terminal being grounded.

In operation, the corona discharge device 60 is adapted to provide anegative electrostatic charge onto the surface 12 of the recordingelement 16 through the screen 62. The electrode 72 is maintained at avoltage of at least 5,000 volts with respect to ground; the resistor 48is adjusted to provide a negative bias of between 0 and 500 volts to thehousing 64, and the variable voltage source 50 is adjusted to providevoltage of between 0 and +400 volts to the support plate 20. When thusenergized, a corona discharge is directed onto the surface 12 bytheelectric wind produced, providing the surface 12 with a uniform negativeelectrostatic charge. Because the optical transmission of the screen 62is less than 50 percent, most of the ultraviolet light emanating fromthe knife edge 78 of the electrode 72 is substantially filtered(blocked) so that only a relatively small amount of the ultravioletlight reaches the surface 12. The values of voltage and dimensions givenherein are not critical. They are merely illustrative and are not to beconstrued in a limiting sense.

The corona discharge device 60 is provided with means to convert ozoneformed during the corona discharge to oxygen in a manner similar to thatdescribed for the corona discharge device 10. Thus, the inner surface ofthe enclosed housing 64 is coated with a catalytic coating 54 of one ofthe aforementioned catalytic materials that catalyzes the chemicalreaction wherein oxone is converted to oxygen. Both of the screens 62and 70 are either made of, or coated with, a catalytic material of thetype described, such as silver, gold, nickel, platinum, iron, or theiralloys, nickel sesquioxide, or copper oxide, for example.

Some catalytic materials 82, in the form of compounds, such as bariummonoxide and silver peroxide, that do not lend themselves easily tobeing applied in the. form of the catalytic coating 54 may be disposedwithin the interior 74 of the housing 64 in foraminous screen containers84. The screen containers 84 may be secured, by any suitable means, tothe insulating rods 76, for example. The screen containers 84 may berefilled if the catalytic materials 82 should become poisoned in timeand with use.

A fan 86 is disposed by any suitable means adjacent to the screen 70 andadapted to rotate in a direction, when suitably energized, to cause airto flow gently through the housing 64 in a direction indicated by thearrows 88. Thus, the fan 86 comprises air moving means to cause ozone toflow through the catalytic materials 82, wherein the ozone is convertedto oxygen, as well as means to prevent ozone from reaching the surface12 to be charged. Although the fan 70 is illustrated in FIG. 3 as beingoutside of the housing 64, it may be included within the housing 64 anddisposed to cause air to flow from the interior of the housing 62 outthrough the screen 70, in the direction of the arrows 88.

While the novel corona discharge device and 60 have been described incircuits wherein negative corona discharges were produced, it is alsowithin the contemplation of the present invention to reverse thepolarities of the voltage sources so that the novel corona dischargedevices 10 and 60 produce positive corona discharges.

Iclaim:

1. Apparatus, adapted to operate in air, for applying an electrostaticcharge to a surface, said apparatus comprising:

an electrically conductive housing comprising a foraminous screen as aportion thereof, said screen being adapted to be disposed adjacent saidsurface to be charged,

a corona generating electrode enclosed within said housing and insulatedtherefrom, said electrode being disposed adjacent to said screen andadapted to generate, when energized, a corona discharge in the generaldirection of said screen, said corona discharge producing ozone in air,and

a catalyst within said housing and spaced from said corona generatingelectrode for converting said ozone to oxygen.

2. Apparatus, adapted to operate in air, for applying an electrostaticcharge to a surface as described in claim 1, wherein said screen has anoptical transmission of less than 50 percent.

3. Apparatus, adapted to operate in air, for applying an electrostaticcharge to a surface as described in claim 1, wherein said catalystcomprises a metal selected from the group consisting of silver, gold,nickel, platinum, iron, and alloys thereof.

4. Apparatus, adapted to operate in air, for applying an electrostaticcharge to a surface as described in claim 1, wherein said screencomprises a catalyst for converting ozone to oxygen, said catalystcomprising a metal selected from the group consisting of silver, goldnickel, platinum, iron, and alloys thereof.

5. Apparatus, adapted to operate in air, for applying an electrostaticcharge to a surface as described in claim 1, wherein said catalystcomprises a metal oxide selected from the group consisting of nickelsesquioxide, barium monoxide, cupric oxide, and silver peroxide, and

means within said housing containing said metal oxide so that said ozonemay come in contact with said metal oxide.

6. In apparatus, adapted to operate in air, for applying anelectrostatic charge to a surface, wherein a corona generating electrodeis enclosed within an electrically conductive housing, the improvementcomprising:

a foraminous screen comprising a portion of said housing, said screenbeing adapted to be disposed adjacent said surface to be charged, saidscreen having an optical transmission of less than 50 percent;

said electrode comprising an elongated strip of metal elongated in adirection substantially parallel to said surface, formed with a knifeedge pointing in the general direction of said screen, said electrodebeing adapted to generate when energized, a corona discharge with anelectnc win in the general direction of said screen, said coronadischarge producing ozone in air, and

a catalyst spaced from said electrode and cooperatively associated withsaid housing for converting said ozone to oxygen.

7. In apparatus, adapted to operate in air for applying an electrostaticcharge to a surface, as described in claim 6,

said catalyst comprising an oxide selected from the group consisting ofnickel sesquioxide, barium monoxide, cupric oxide, and silver peroxide;

foraminous means disposed within said housing for containing saidcatalyst,

a second foraminous screen, spaced from said first-mentioned foraminousscreen, and comprising another portion of said housing; and

air moving means cooperatively associated with said housing for movingozone within said housing through said second screen.

2. Apparatus, adapted to operate in air, for applying an eleCtrostaticcharge to a surface as described in claim 1, wherein said screen has anoptical transmission of less than 50 percent.
 3. Apparatus, adapted tooperate in air, for applying an electrostatic charge to a surface asdescribed in claim 1, wherein said catalyst comprises a metal selectedfrom the group consisting of silver, gold, nickel, platinum, iron, andalloys thereof.
 4. Apparatus, adapted to operate in air, for applying anelectrostatic charge to a surface as described in claim 1, wherein saidscreen comprises a catalyst for converting ozone to oxygen, saidcatalyst comprising a metal selected from the group consisting ofsilver, gold nickel, platinum, iron, and alloys thereof.
 5. Apparatus,adapted to operate in air, for applying an electrostatic charge to asurface as described in claim 1, wherein said catalyst comprises a metaloxide selected from the group consisting of nickel sesquioxide, bariummonoxide, cupric oxide, and silver peroxide, and means within saidhousing containing said metal oxide so that said ozone may come incontact with said metal oxide.
 6. In apparatus, adapted to operate inair, for applying an electrostatic charge to a surface, wherein a coronagenerating electrode is enclosed within an electrically conductivehousing, the improvement comprising: a foraminous screen comprising aportion of said housing, said screen being adapted to be disposedadjacent said surface to be charged, said screen having an opticaltransmission of less than 50 percent; said electrode comprising anelongated strip of metal elongated in a direction substantially parallelto said surface, formed with a knife edge pointing in the generaldirection of said screen, said electrode being adapted to generate, whenenergized, a corona discharge with an electric wind in the generaldirection of said screen, said corona discharge producing ozone in air,and a catalyst spaced from said electrode and cooperatively associatedwith said housing for converting said ozone to oxygen.
 7. In apparatus,adapted to operate in air for applying an electrostatic charge to asurface, as described in claim 6, said catalyst comprising an oxideselected from the group consisting of nickel sesquioxide, bariummonoxide, cupric oxide, and silver peroxide; foraminous means disposedwithin said housing for containing said catalyst, a second foraminousscreen, spaced from said first-mentioned foraminous screen, andcomprising another portion of said housing; and air moving meanscooperatively associated with said housing for moving ozone within saidhousing through said second screen.